High strength newsprint

ABSTRACT

HIGH STRENGTH NEWSPRINT IS MADE BY CALENDERING A NEWS PRINT PAPER WEB BETWEEN A SMOOTH HARD METALLIC SURFACE AND A FIRM RESILIENT SURFACE SO THAT THE THICKNESS OF THE PAPER UNDER THE APPLIED PRESSURE VARIES FROM POINT-TOPOINT IN APPROXIMATE PROPORTION TO THE CORRESPONDING POINT-TO-POINT BASIS WEIGHT OF THE PAPER. THE CALENDERING IS EFFECTED SO THAT SMOOTHING PRESSURE IS APPLIED BY PASSING THE WEB THROUGH AT LEAST TWO RESILIENT NIPS, EACH NIP COMPRISING A HARD ROLL AND AN OPPOSING RESILENT ROLL, SO THAT EACH SIDE OF SAID WEB CONTACTS A HARD ROLL AT LEAST ONCE. THE NEWSPRINT PAPER MAY HAVE AT LEAST 25% OF SUPERGROUND WOOD.

w. G. MlHELlCH 3,759,785

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Sept '13, 1973 w. G. MIHELICH HIGH STRENGTH NEWSPRTNT 5 Sheets-Sheet 3Filed Oct. 6, 1971 United States Patent Oflice 3,759,785 Patented Sept.18, 1973 Int. (:1. D21f 13/00; D21h 5/12 US. Cl. 162-142 11 ClaimsABSTRACT 01; THE DISCLOSURE High strength newsprint is made bycalendering a newsprint paper web between a smooth hard metallic surfaceand a firm resilient surface so that the thickness of the paper underthe applied pressure varies from point-topoint in approximate proportionto the corresponding point-to-point basis weight of the paper. Thecalendering is effected so that smoothing pressure is applied by passingthe web through at least two resilient nips, each nip comprising a hardroll and an opposing resilient roll, so that each side of said webcontacts a hard roll at least once. The newsprint paper may have atleast 25% of superground wood.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application Ser. No. 860,022, filed Sept. 22,1969, and abandoned.

BACKGROUND OF THE INVENTION This invention relates to the manufacture ofnewsprint paper.

DESCRIPTION OF THE PRIOR ART The newsprint manufacturing processinvolves flowing a dilute water suspension of wood pulp fibers on to atravelling, open meshed, fine wire screen (or between two such screens)through which the bulk of the water drains. Further water is usuallyremoved by using a vacuum to draw it from the layer of fibers formed onthe screen. The layer of fibers (then a wet, weak piece of paper) ispassed through the nips formed by paired opposed rolls to squeeze outmore water and is then passed over rotating, heated cylinders to remove,by evaporation, most of the remaining water content to yield a dry sheetof paper usually containing 5% to 15% moisture. At this point in theprocess newsprint paper typically has a weight, called the basis weight,of about 32 pounds per 3,000 square feet of area: it will have athickness (caliper) of about 0.0055 to 0.0060 inch.

The paper at this stage has surfaces too rough to give a well-printednewspaper, by the commonly used process. Also, due to its soft, bulkystructure (high caliper) paper rolls made from it would be of low weight(relative to diameter), soft and easily damaged in handling or transit.

These shortcomings are presently remedied by the step of calendering inwhich the paper is passed through successive nips formed betweenrotating, heavy iron rolls stacked upon one another with their axesparallel and in a common vertical plane. In this commonly used calenderstack, the weight of each roll is supported by the one below. The paperpasses through the nips from top to bottom of the stack and is subjectedto step-wise increases of pressure due to the increase in the number ofrolls weighing down upon each successive nip. In its passage throughthis series of nips the paper is progressively reduced in thickness andits surfaces made smoother by compression and a moderate frictionalironing effect.

After the calendering process, newsprint will be typically about 0.0031to 0.0033 inch thick and will have a surface smooth enough to give goodreproduction in the printing process. It will also have the densityrequired to make a hard, sound roll of paper which can adequately resistthe stresses of handling and shipping.

It is an essential property of newsprint that it should run throughprinting presses with a minimum number of breaks of the paper web. Theaverage tensile strength of newsprint is several times more than thenormal tensile loadings imposed upon it by the printing presses. Inconsequence, paper breaks in printing presses are infrequent and maytypically occur with a frequency of once for every 50 to 500 miles ofpaper printed. Even at these low frequency levels, paper breaks are acause of troublesome delays and the resulting down time of the machineryadds significant costs to the printing of newspapers.

It is now established that most press breaks result from the presence inthe web, specially at or near the edge of the web, or flaws or weakspots. These flaws are most commonly associated with the presence ofshives in the sheet. Shives are small bundles of wood fibers which arenot separated from one another in the pulp making process, and remainedas small wood slivers. They may vary in size from innocuous aggregationsof two or three fibers up to clusters of a hundred or more fibers. Theymay range up to half an inch long and have widths and thicknesses up to,or over 0.02 in.

As more fully described below, the applicant has found that shives havevery little, or no, effect upon the strength of uncalendered newsprintand that their weakening effect in calendered newsprint arises almostwholly as a direct consequence of the present conventional mode ofcalendering between hard, iron rolls. The applicant has also found thatconventional calendering results in the development of numerous weakspots in the paper in non-shive bearing areas where small, chanceaggregations of fibers give spots of high basis weight which are crushedin calendering. The present invention provides a method for effectingthe changes now sought by the conventional calendering process whilesubstantially reducing the loss of strength caused by conventionalcalendering.

The technical literature on calendering newsprint deals largely withmechanical variables related to smoothness and caliper changes in thepaper. It has been recognized that the conventional newsprintcalendering process reduces the strength of shive bearing areas. Therehas been no known prior recognition that this process results in seriousstrength loss in those small non-shive bearing areas where, by chancedistribution of fibers within the web, basis weights are substantiallyhigher than average. The applicant has discovered that such areas, whenconventionally calendered form within the sheet a multitude of weakspots which substantially weaken the sheet and the applicant hasprovided a process by which the weakening of such high weight spots, andof shive bearing areas, is much reduced.

SUMMARY OF THE INVENTION It is an object of the invention to provide amethod by which newsprint paper, as produced after the dryer section ofthe paper machine, may be reduced in thickness, or calendered, to thelesser caliper wanted in the final product. The invention makes itpossible to increase the surface smoothness of the paper simultaneouslywith its reduction in caliper and to give, for any degree of reductionin caliper, a smoother surface than possible for the like degree ofreduction by conventional calendering. The invention provides thefurther and distinguishing feature of reduced caliper and increasedsmoothness with a much reduced loss of strength, as compared to the losscaused by conventional calendering.

In the manufacture of newsprint using the conventional calenderingprocess, it is necessary to use a fiber furnish which will providesufiicient initial strength that there will remain, after the strengthdamaging action of conventional calendering, sufiicient residualstrength to permit the paper to be printed with no more than anacceptable level of press breaks. In general, the measures used toprovide this strength all involves higher costs. Thus, for instance,increased strength may be had by increasing the proportion of chemicalpulp and reducing the proportion of groundwood used. Depending on thetype of chemical pulp involved, it may cost 1.5 to over 3 times the costof ground-wood and thus any increase in its proportion substantiallyincreases the cost of the newsprint. In another alternative, thegroundwood content can be increased in strength by increasing the energyused to grind it. Several hundred kilowatt hours extra may be requiredper ton of pulp to give a substantial strength increase. This representsa substantial cost increase and, since grinder motor capacities arealways limited, reduces the amount of groundwood which can be produced.Since the method of the invention substantially reduces the strengthloss on calendering, finished paper of equivalent strength to that madewith conventional calendering can be made from paper designed to have alower strength in the pre-calendered condition with consequent savingsin cost of chemical pulp and/ or grinding energy.

The conventional calendering process is a continuous one in which thepaper is pressed between metal surfaces in the nips formed betweenopposed parallel, rotating rolls. Its ancestor, a batch process,obtained very similar results by applying pressure to a stack made up ofsheets of uncalendered paper between each of which was placed a metalplate having two smooth surfaces. This batch form can serve toillustrate the cause of the paper strength loss caused by conventionalcalendering and to clearly show both how the present method ofcalendering differs from it and why the present method provides reducedloss of strength.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherillustrated by reference to the accompanying drawings, in which:

FIG. 1 is a graph showing the distribution of areas of differing basisweight in a sheet of newsprint paper;

FIG. 2a is a cross-section on an exaggerated scale of a typical sheet ofuncalendered newsprint paper;

FIG. 2b is a cross-section on the same scale as FIG. 2a of the samesheet of newsprint paper after conventional calendering;

FIG. 3 is a schematic cross-section on an exaggerated scale representingthe calendering effect of pressing a newsprint sheet between the flatsurfaces of opposed metal plates;

FIG. 4 is a similar view of FIG. 3 showing the pressing of a newsprintsheet between a flat metal surface and a flat resilient surface, being abatch embodiment of the present invention; and

FIG. 5 is a schematic vertical cross-section through an apparatussuitable for the continuous operation of the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the curve marked Agives the frequency of occurrence of various basis weights within asample of commercial calendered newsprint. The individual samples onwhich basis weights were determined were randomly selected areas ofapproximately one ten-thousandth 4 of a square inch. Such samples aretoo small for the conventional method of measuring basis weight bydirect weighing and measurement of surface area. Basis weights weredetermined by X-raying the paper, measuring the optical density of smallareas of the X-ray film and converting the density values to basisweights by appropriate calibration procedures. FIG. 1, curve A, showsthat, while the average basis weight of the paper is about the normal 32pounds per ream of 3,000 square feet, this figure is the result ofaveraging the basis weights of very large numbers of small areas havinga wide range of basis weights. Curve A shows that 40% are below 30pounds while 5% are above 39 pounds.

In FIG. 2a shown a simulated cross section of a piece of uncalenderednewsprint in which the spacing between the upper surface 11 and thelower surface 12 of the paper is in proportion to the basis weight asshown by the scale at the left. In effect, FIG. 2a is a simulation ofpart of the data from which curve A of FIG. 1 was obtained.

FIG. 3 shows, as 13, the paper segment of FIG. 2 as held under pressure,indicated by the arrows 16 between the smooth surfaced, metal plates 14and 15 in the course of being calendered by the batch form of theconventional calendering process as previously described. It is to beparticularly noted that the segment 13 has now been so compressed as tomake its upper surface 11 and lower surface 12 plane and parallel.

FIG. 3 can equally serve to represent conditions in the continuous formof the conventional calendering process: here 14 and 15 would representparts of adjacent metal rolls and the paper 13 would be in the conditionin which it is found at the narrowest part of the nip between the rolls.Again, the two surfaces of the paper are parallel. The compression ofthe paper as illustarted in FIG. 3 results in the development, in thesheet under pressure of density differences since the thickness isuniform While the substance per unit area varies as per FIG. 2a. Thesedensity differences are represented in FIG. 2b in which the height ofthe section at any point represents the density at that point.

Particular attention is drawn to the peaks in FIG. 2b marked 17 and 18.Theoretically, in pressing these will have attained maximum densities of1.63 and 2.52 gms./ ml. respectively. These densities are greater thanthe ultimate density of the solid material of which newsprint is madeand therefore cannot be reached in calendering. Instead, part of thevolume of these peaks (indicated by the horizontal hatching) must flowlaterally to occupy space as indicated by the dashed lines 19 embracingthe displaced volumes shown by diagonal hatching.

The forces required to compress and laterally displace the materialunder high weight peaks, such as 18, crush and cut the fibers stackedthere to form the weight peak. Additionally, the lateral flow of massalso results in fiber breakage. In sum, high weight, small areas ofpaper, which should be stronger because they contain more material, areactually weaker because of damage caused by the conventional calenderingmethod. In general, the higher the basis weight of a small, localizedarea, the lower will be the strength of that area. It is conventionaldoctrine that higher basis weight paper will be stronger than that oflower basis weight. This, despite the foregoing is still true withrespect to averaged strength over larger areas of paper. However, withinthe larger areas, small areas of higher than average basis weight willhave lower strength than those of average basis weight in the case ofthe papers with both higher and lower average basis weights.

The same paper, for which the basis weight distribution of random smallareas is given by curve A of FIG. 1, was tested for tensile strength.The 15 mm. wide strips used in the tensile test were matched with theX-ray films previously made and the exact locations of the tensilebreaks in the strips were noted on the corresponding X-ray films. Theaverage basis weight of the paper along a strip 0.3 mm. wide andcentered on the break lines was measured by film densitometry. Thedistribution of basis weights so found is given by line B of FIG. 1. Itwill be noted that while of the area of the paper had a basis weightbelow 31 pounds (curve A), only 2% of the area of the zones throughwhich tensile failure occurred were as low. Also, while 50% of the paperthrough which tensile failure lines ran had a basis weight over 39.5pounds, less than 5% of each of the strips tested for strength was thatheavy. It is thus obvious that the lines of tensile failure selectedsections of high basis weight. The apparent anomaly is due to theincreased calender damage done to higher basis weight areas by theconventional calendering process, as described.

Shives in paper represent small areas in which the normal basis weightof the paper is supplemented by the basis weight contributions of theshives. They are, therefore, areas of high basis weight and are subjectto the weakening action of calendering as described. It will thus beobvious that the weakening of shive-bearing areas by conventionalcalendering is only a particular case of the general case ofconventional calendering damage to small The comparative effects ofconventional calendering and of calendering by the present process wereclearly shown in a test in which a lot of paper was made from a normalnewsprint furnish of groundwood and sulphite pulps; a second lot ofpaper was made from the same pulp blend to which a small proportion ofshives had been added to develop added small areas of high basis weightin the second lot of paper. The two lots were divided: part of each wastested for tensile strength; part of each was calendered by theconventional method to 0.0032 inches thickness then tested for tensilestrength; another part of each was calendered to like thickness by theapplicants method and was then tested for tensile strength. Theresulting data are given in Table I.

These tensile tests, and others referred to in this application weredone according to'TAPPI Standard tensile test for paper T404-ts 66.

areas of high basis weight which the applicant has discovered.

The newsprint calendering method of the invention achieves the desiredreduction of paper caliper and the desired increase in surfacesmoothness while very substantially reducing the number, and theweakness, of the weak spots in the paper relative to conventionalcalendering.

The principle of the present method is represented in FIG. 4 showing itsapplication as a batch method in analogy to the conventional process asrepresented in FIG. 3. As in FIG. 3, 14 and 15 represent smooth surfacedmetal plates to which pressure is applied as indicated by the arrows 16.Below the upper plate 14 is placed a firm, resilient sheet which may bemade, for instance, of a fairly hard rubber composition. The paper beingcalendered is represented by 13 and its two surfaces are represented by11 and 12.

An important feature of the present method of calendering newsprint isthe interposition of a firm resilient material between the paper beingcalendered and one of the hard surfaces through which pressure isapplied to the paper to effect the calendering. One importantconsequence of interposing this resilent layer is indicated by theprofile of the paper surface 11 as indicated in FIG. 4.

Since the surface of the layer 20 is resilient, small areas of highbasis weight in the paper can now indent into the layer 20 and the uppersurface 11 of the paper is no longer forced into a single plane as isthe surface 11 with conventional calendering. By the present process,the thickness of the paper from point-to-point under the calenderingpressure can vary in some relationship with the correspondingpoint-to-point variation in basis weight. Thus a high basis weight areacan indent a substantial distance into the resilient layer 20 as shownat 21 of FIG. 4. Whereas in conventional calendering very high pressuresare brought to bear to force high basis weight areas to a common caliperwith the rest of the paper with resulting crushing and loss of strengthas described, the applicants process limits the extra pressure placed onsuch areas by permitting them to indent the resilient surface. Inconsequence there is a very substantial reduction in the number ofdamaged areas when the applicants process is substituted for theconventional one. With fewer damaged areas, average paper strength ishigher and more uniform from area to area within the paper.

It will be seen that the addition of shives has had only a minor effectupon the strength of the uncalendered paper. Calendering by theconventional process has caused a loss of about 18% in the averagestrength of the paper without added shives and a loss of 26% in thatwith added shives. In contrast, calendering by the applicants methodcaused corresponding losses of only 3% and 9%, respectively.

Many miles of newsprint ordinarily run through a printing press betweeneach paper break. It is therefore obvious that its average strength,even with conventional calendering, is more than adequate to support thestresses imposed upon it. Only the rare, weakest parts of newsprint arelikely to cause a break. The standard deviation of tensile tests providea masure for roughly estimating the frequency with which various levelsof strength may be expected. Statistically tensile tests of averageminus three times standard deviation may be expected about twice inevery thousand tests. The last row of Table I gives such values foruncalendered paper, conventionally calendered paper, and papercalendered by the applicants process both with and without added shives.It will be noted that standard deviations are higher for conventionalcalendering than for the applicants calendering process and as a resultthese important, infrequent test values are at a very much higherstrength level for newsprint paper calendered by the present processthan for paper calendered in the conventional way. Conversely, if somespecific, low, tensile strength value is required before a newsprint webwill break in the printing press, such values will occur with muchreduced frequency in paper calendered by the applicants method ascompared to paper calendered by the conventional method.

Referring to FIG. 4, it will be seen that the paper surface 12 is beingpressed into conformity with the smooth, plane surface of the metalplate 15. This action substantially increases the smoothness of thepaper surface 12 and fits it for faithful reproduction of printing. Thesurface 11 contacts the smooth but resilient surface of the layer 20 andis not made as smooth as the surface 12. The applicant therefore findsit desirable, in order to get good printing surfaces on both sides ofthe paper, to repeat the calendering process with the paper face 11' incontact with the surface of the plate 15 in the second instance. Thisdouble calendering produces good, and equal smoothness on both sides ofthe paper.

The applicant has described the calendering process of the invention interms of its application as a batch process to more clearly set forthits principles, benefits and causes of the improved results given by it.Commercial application requires that the process be adapted tocontinuous operation. FIG. is a schematic cross-section of an apparatuswhich may be used to apply the process of the invention to thecalendering of the newsprint. In FIG. 5 the number 22 representsuncalendered paper moving in the direction indicated by the arrow 23into the nip formed between the roll 24 and the roll 25 about which itis wrapped. Roll 24 has a journaled metal core bearing a resilientrubber or plastic cover 26. Means, not shown, are provided for pressingthe roll 24 against the paper in the nip which is supported by roll 25.Means are provided by which the pressure between the rolls 24 and 25 maybe adjusted to effect, in combination with the like action of the otherrolls shown, the degree of caliper reduction required. The rolls 27, 28and 29 have the same structure and functions as the roll 24. The roll 25has a hard metal shell having a smooth, outer cylindrical surface whichsmooths and polishes the surface of the paper pressed against it. Meansmay be provided by which the roll 25 may be heated to thereby heat thepaper, increase its plasticity and render it more responsive to thecalendering action. Such heating means may be located either within oroutside the cylinder 25, or both. After passing through the pressure nipformed between the rolls 25 and 27, the paper continues through nipsformed between rolls 28 and 30 and 29 and 30 to complete the calenderingprocess. Roll 30 is of the same general nature as roll 25. Thecalendered paper 31 continues to a reel-up mechanism not shown. It willbe noted that the applicants calendering process thus involves twoseparated but similar calendering steps. In the first step, one surfaceof the paper is calendered in contact with the smooth hard surface ofthe roll 25. In the second step, the opposite surface of the paper iscalendered in contact with the smooth hard surface of the roll 30.

Means are provided for driving rolls 25 and 30 while rolls 24, 27, 28and 29 are usually driven by rolls 25 and 30 by frictional forcestransmitted through the paper. Apparatus suitable for performing theprocess is described in US. Patents 3,365,774 ('Kusters) and 3,124,504(Mahoney et al.).

While the apparatus described by Mahoney et al. can be used for theperformance of the present process, it should be noted that there is noteaching of the applicants process by Mahoney et al. It is a primarypurpose of the method of the present invention to effect a substantialreduction in the caliper of the newsprint while simultaneouslyincreasing surface smoothness. The caliper (thickness) reductions to beobtained by the applicants process ranges from 25% to 50% of thethickness of the uncalendered paper. Mahoney et al. provide a method andapparatus specifically designed to smooth, gloss and coalesce the papersurface without substantially affecting the bulk or thickness of the web(US. Pat. 3,124,504, col. 1, lines 58 and 59) and this emphasis onlittle or no change in paper thickness is repeated at numerous places intheir disclosure. Furthermore, in setting out the advantages of theirmethod and apparatus over other calendering means, Mahoney et al. cite,as a disadvantage of these, that they produce an end product thethickness or bulk of which is altered considerably as a result thereof(col. 1, lines 52 and 53). Thus, their process does not teachsubstantial caliper reduction, which is an object of the presentinvention. Furthermore, the effects of smoothing and glossing sought byMahoney et al. are the result of a differential velocity of paper andhard polished roll through the pressure nip, a characteristic of nipsformed between rolls of different hardness. The paper surface is draggedover and smoothed by the metal surface and this relative slippage isessential to the production of smoothing. The applicants process is, asdescribed with respect to FIG. 4, used without differential velocitiesbetween the paper and the metal surfaces. Such conditions will not givethe glossing and polishing which are the objectives of the invention ofMahoney et al.

Due to the deformation of the resilient roll cover 26 in its passagethrough the pressure nip, its surface velocity changes frompoint-to-point through the nip relative to the constant surface speed ofthe roll 25. Since the paper adheres more to the cover 26 than to thepolished surface of the roll 25, the differential velocity of the cover26 results in the paper sliding slightly on the polished surface 25. Incombination with the pressure exerted between the rolls 24 and 25, thissliding effects an ironing action which is very efficient in smoothingthat surface of the paper in contact with the surface of roll 25. Afterpassing the roll 27, the paper will be smoother on the side 32 than onthe side 33. To eliminate this difference the paper is recalenderedthrough the nips formed by rolls 28 and 30, and 29 and 30 whereby thesurface 33 benefits in turn from the described ironing process and thesurfaces 32 and 33 become of substantially equal smoothness.

The newsprint calendering process of the invention may be applied touncalendered newsprint of moisture content up to 12 to 15%. The metallicsurface forming one element of the calendering nips is preferably soheated as to give paper temperatures in the nips in the range from to200 F. Actual temperature of the metal rolls to give such papertemperatures will depend upon such factors of specific equipment unitsas operating speed, mode of heating and duration of contact of the paperweb with the heated surface. Suitable pressures for calendering will liein the range of 100 to 1,000 pounds per lineal inch of nip width. Thecovering 26 of the rolls 24, 27, 28 and 29 may be of hard rubber orother elastomer with a preferred hardness in the range of 0 to 25 asmeasured by the Pusey and Jones /s" ball method with a preferred rangefrom 2 to 7.

These parameters of temperature, roll hardness and nip pressure, papermoisture and speed are the usual variables of any paper calenderingprocess and the proper combination of them to achieve a desired degreeof calendering will be within the range of adjustments available on thepaper making machine and calender, and within the competence of papermachine operators. A desired degree of calendering would involve asubstantial reduction of paper thickness. Ordinarily this would be froman initial thickness of 0.005 inch to 0.0065 inch down to 0.003 to0.0038 inch.

The process for making newsprint, which is the subject of thisapplication may be applied to papers made of any of the known blends offibers used for making newsprint. The applicant has found that in commonwith all newsprint papers is the fact that the strength losses producedby conventional calendering are a consequence of the application ofexcessive pressures upon localized areas of high basis weight and thatsuch areas exist in newsprints made from all types of fiber furnishesand that such areas are not necessarily the result of the presence ofshives, but may be due to random excesses of normal fibers in smallareas. The blends used for making newsprint may, for example, be made ofstone or refiner groundwood made from suitable softwood or hardwoodspecies, in combination with varying proportions of chemical pulps (orwithout them) such as semibleached kraft, high or normal yield sulphitepulps, or others.

Independent of the particular combination of fiber types used in makingnewsprint, the wet forming processes used invariably result in theformation of numerous small areas of basis weight substantially aboveaverage basis weight. Newsprint paper is normally made at a basis weightof 32 pounds per 3,000 square feet. This weight per unit area is roughlyequivalent to that given by three of the wood fibers used in making thepulps from which the paper is made. Actually, in the groundwood, whichis the principal complement of newsprint, most of these fibers have beenbroken into fragments and a typical section through the thickness of thepaper will consist of many more than three fibrous elements. However,there are many fibers and fiber fragments of the full cross sectionalthickness of the original wood fiber and many of these will cross and besuperimposed at many locations within the paper. Given that the finalarrangement of fibers in the paper is largely a matter of chance, therewill be many small areas in which the number of fibers through thethickness of the paper is other than the average of three. It may beless or it may be four, five, six, or more, with reduced likelihood ofoccurrence being associated with the increased number of superimposedfibers.

Since each superimposed fiber is equivalent to about pounds in basisweight, six of them would represent a very small area having a basisweight of about 60 pounds or nearly twice the average basis weight.

Examples of newsprint to which the invention is specially applicable arethose containing supergroundwood, as will now be explained.

Newsprints are generally made with mixtures of stone groundwood pulpsand chemical pulps. The relatively inexpensive stone groundwood pulpsare made by forcing whole logs of wood against a revolving, abrasivegrindstone, while the chemical pulps require expensive chemicaltreatment of the wood and yields are much lower in the latter case. Forthe sake of economy, newsprint producers have, for many years, beentrying to develop methods which result in the production of newsprintwith a minimum content of chemical pulp.

More recently, a process for making groundwork by disc-refining woodchips instead of forcing whole logs of wood against a revolving*grindstone has been developed. This refiner-groundwood pulp, whenproduced under known optimum operating conditions, is of higher strengththan the stone-produced pulp and drains well on the webfonming sectionof a paper-making machine even in the absence of chemical pulp. Becauseof its superior strength qualities, this pulp is often calledsupergroundwood. It has been found to have sufficient wet-web strengthto run on a paper machine with acceptably few breaks, and, is,therefore, starting to find more and more applications as a replacementof all or part of the stone groundwood component used in newsprint pulpfurnishes. This topic is well covered in Pulp and Paper Magazine ofCanada, vol. 64, No. 7, T299 (July 1963), supergroundwood: ItsManufacture from Chips and Use as Sole Newsprint Furnish, L. R. Beathand M. T. Neill.

However, this supergroundwood pulp was also found to have a higherspecific volume than the stone groundwood pulp (Science and Technologyof Mechanical Pulp Manufacture, N. G. Gavelin, Lockwood Publishing Co.(1966), p. 213). Furthermore, any reduction in the proportion ofchemical pulp content in the furnish also results in a higher paperspecific volume because the chemical pulp has a lower specific volumethan the groundwood pulp. The increase in specific volume due to theabove reasons leads to difiiculties meeting newsprint smoothness andbulk specifications and requires heavier calendering on the papermachine. See the Gavelin article.

The heavier calendering of newsprnt made from supergroundwood pulp togive normal newsprint thickness results in a reduction of its averagestrength. Furthermore, supergroundwood generally contains more longshives than ordinary stone groundwood as explained in Science andTechnology of Mechanical Pulp Manufacture, N. G. Gavelin, LockwoodPublishing Co. (1966), p. 212. The presence of shives, coupled with theheavier calendering introduces more flaws or weak points in thenewsprint web. It has been shown by G. F. Sears, R. F. Tyler, G. W.Denzer, Shives in Newsprint. The Role of Shives in Paper Web Breaks,Pulp and Paper Magazine of 10 Canada, vol. 66, No. 7, T-351 (July 1965)that these flaws trigger newsprint web breaks on printing presses Forthese reasons it is not possible, according to normal newsprintmanufacturing procedure, to take full advantage of the higher strengthof supergroundwood by substantially reducing the proportion of chemicalpulp.

An aspect of the present invention is, therefore, the combination ofmaking paper with a newsprint pulp furnish of which all of themechanical pulp content is supergroundwood pulp and calendering thepaper made therefrom in a resilient nip calender as described herein.The same benefits to a lesser degree may be obtained by making paperwith a newsprint pulp furnish of which about one-quarter or more byweight of the mechanical pulp content is supergroundwood, andcalendering the paper made therefrom in a resilient nip-calender.Despite the reduced proportion of chemical pulp, compared with normalpractice, the resulting paper still has adequate strength to run onhigh-speed printing presses with acceptably few breaks.

A preferred aspect of the invention, therefore, is to employ a newsprintfurnish made up entirely of supergroundwood pulp. The invention alsocontemplates the use of a varying amounts of chemical pulp depending onthe requirements of the newsprint user. In any case, the combination ofthe use of a proportion of not less than 25% supergroundwood pulp, basedon the total mechanical pulp, in combination with resilient-nipcalendering per mits the amount of chemical pulp employed to be reducedbelow that which would have to be used to make an acceptable newsprintusing a paper made from a furnish in which all the mechanical pulp isgroundwood pulp and metal-nip calendering.

This aspect of the invention contemplates making supergroundwood pulpwith disc refiners and auxiliary equipment, making paper with this pulp,and, after drying, calendering the dried paper in a resilient-nipcalender. The presence of the supergroundwood pulp, in amounts of atleast about a quarter by weight of the total mechanical pulp, allows areduction in chemical pulp content without deterioration of the papermachine runability. The res1l1ent-nip calendering prevents thedeterioration in dry strength and generation of shive flaws which wouldresult from heavier conventional calendering required to reduce thebulkier supergroundwood furnish to normal newsprint thickness.

The production of supergroundwood pulp is an art which is stillevolving, but some preferred methods have now been established. Forexample, the applicant has produced excellent supergroundwood pulps bydisc-refining softwood chips in one pass through Sprout Waldronsingle-disc refiners or through Bauer double-disc refiners. Enoughdilution water is added so as to give a consistency of about 15% to 40%in the refining zone. About to about H.P.-days per oven-dry ton ofenergy is appl ed at the refiners, equipped with refining plates of asuitable pattern (for example D 13A001 on the Sprout and 40106 on theBauer).

The pulp is then screened and centricleaned as usual. Although there areseveral possible variations to this process for the production ofsupergroundwood, it is not always possible to make supergroundwood withrefiners unless specific conditions of equipment and operation arecorrectly combined to get it. For example, the disc-refining of the woodchips can be done in one, two, three or more stages and it can be doneunder atmospheric or pressurized conditions. Sometimes the consistenciesin the second and subsequent stages can be lowered to about 5% andsupergroundwood quality still maintained. This process, in any of theforms mentioned, results in supergroundwood pulps which are of superiorstrength and papermaking characteristics than stone groundwoods.

A comparison of typical properties of supergroundwood, stone groundwoodand sulphite pulps made with northern softwoods is given in Table H.

1 1 1 2 TABLE H Nip pressure at each nip: 350 p.l.i.

s St N Resilient roll covering plastometer, P & J units: 3 333: gg? $33Smooth non-resilient roll surface temperature: 320 F.

wood wood p t Paper moisture after calendering: 10.2% o.d.2223353.?iltfifittllfiitifiittiiistraj: 2. 92 2. 22 1. 5 Othercombinations of P r moisture content. metal T.A.P.P.I. burst factor-.-1s 14 53 roll surface temperatures and lmeal 111p pressures may be 80 459 chosen to produce the caliper and smoothness of newsprint desired,without losing the dry strength of the super- A specific example of theconditions and equipment 10 groundwood and without the generation ofnumerous shive required to produce supergroundwood according to oneflaws. preferred method is given below: The surprising differencebetween the average strength wood fir 2222:23335323223322.? Chipquality: As for newsgrade sulphite production ni calender i Shown in th0110 in a 651 ml Type of refiner: Sprout, Waldron 42-1 with 2000 H.P. ps e W g a e motor ABLE III Plate pattern: D 13 A 00l/N1-hard T Rate ofchip feed: 21 oven-dry tons per day ggg fi Rate of addition of dilutionwater to chips: 13 U.S.G.P.M. 0518mm calendel; Temperature of dilutionwater: 40 to 80 F.

Basis we] ht, A.D. lb./ earn 24 36 500 34.7 31.8 Average refiner motorload: 2000 RR Apparent specific voluriie, e/Aii). 1.51 1. 51 Averagespecific energy consumption: 95 H.P.days per TtA-P-P-lburstfflctor gg-gton S t h 17 t t s As mentioned prev1ously, supergroundwood 1s also of25 re 0 Damn a higher specific volume and generally contains more longshives than stone groundwood. With heavier conven- Table IV shows theeffects of conventional and resilienttional calendering of the newsprintmade with supernip calendering on the tensile strengths ofsupergroundgroundwood to obtain normal newsprint calipers, the averwoodstrips having at least one shive lying at right angles age dry strengthof the newsprint is reduced and more to the direction of stressing. weakshive-fiaws are introduced. The invention overcomes Visual observationof the strips used to obtain the above this problem by replacing theconventional calendering results showed that normal calendering causedcuttin of with resilient-nip calendering. the fibers lying directlyabove and below the shives. This Essentially what is required is thatthe newsprint web cutting locally destroyed the fiber network of thesheet leaving the drying section of the paper machine be passed alongthe length of the shive creating, in eiTect, a small through at leasttwo resilient nips, each nip fromed beslit in the paper. This wasconfirmed by the much greater tween a smooth, non-resilient, heatedmetal roll and an number of strips which broke at the shives wtihconvenopposing roll covered with a resilient material such that tionalcalendering than with resilient-nip calendering.

TABLE IV Conventional calendered Resilient-nip calendered Did or Did orShlve did not Shive did not Basis Caliper, length, Tensile, break BasisCaliper, length, Tensile, break wt in. mm. kg./15 mm. at; shive wt. in.mm. kg./15 mm. at shive .00333 9 33.4 .00330 0 2.61 No. 00333 7 32.9.00327 a 2. 35 No. .00323 0 33.2 .00323 7 2. so No. .00320 12 32.6.00330 8 2.60 No. .00333 11 33.6 .00327 13 2. 30 No. .00330 14 32.600330 15 2.60 NO. 00333 11 32.0 .00330 11 2.69 No. 00340 3 33.6 .00337 32.70 NO. 00340 10 33.0 .00330 8 2. No. 00340 7 33.0 .00337 10 2.32 Yes.00327 10 33.2 00343 11 2.21 NO. .00330 7 32.6 00343 10 2. 30 NO. 0033313 32.9 00337 3 2.60 NO. 00313 0 33.0 00330 3 2. 86 No.

Average 32.7 00331 0.73 33.0 00332 0.56 2.63

each side of the newsprint web contacts a metal roll at I claim: leastonce. These resilient-nip calenders are available in 1. A process forproducing newsprint which comprises several dilferent assemblies and aresometimes called Gloss preparing a newsprint pulp furnish of fibers,making a wet Calenders or Burnishers or Thermoplanishers. Suitable paperweb of said furnish, drying said web, calendering machines are describedin US. Patents 3,124,504, 3,190; dried web to reduce its thickness byfrom 25% to 50% 212 and 3,230,867. Although these machines aregeneraland to increase its surface smoothness while causing minily usedto impart very high gloss finishes to papers and Inal strength loss bypressing the dried web between two boards, the applicant has found thatthey can be operated, opposed surfaces one of which surfaces is smoothand according to the invention, to produce normal newsprint made of ahard metallic material and the second of said finishes. For example, theapplicant has used the Beloit surfa c es is made of a firm resilientmaterial such that the Burnisher to calender newsprint in this way. OneSet thickness of the paper between said opposed surfaces un- 0f OperatmgCondltlons 115mg thls eqlllpment W851 der the applied pressure variesfrom point-to-point in ap- Basis weight of newsprint: 32.05 lb./ream (24x 36 x 500) PT Proportion to the correspPnding Point-tori)Oint Papermoisture entering first nip: 16.6% o.d. basls welgbts of the p p removmgsald p p from Speed of paper through calenders: 1350 f.p.m. between saidopposed surfaces and re-pressing it between Total number of resilientnips: 4 two opposed surfaces one of which surfaces is smooth and made ofa hard metallic material and the second of said surfaces is made of afirm resilient material such that the thickness of the paper betweensaid opposed surfaces under the applied pressure varies frompoint-to-point in approximate proportion to the correspondingpoint-topoint basis weights of the paper, said re-pressing beingperformed so that the surface of the paper which was in contact with themetallic surface during the first pressing is in contact with theresilient surface during said re-pressing.

2. A process as defined in claim 1, in which the smooth surfaces made ofa hard metallic material are the surfaces of cylindrical rolls, thesurfaces made of firm resilient material are the surfaces of cylindricalrolls, the said rolls cooperating in pairs of unlike surfaces to formbetween said pairs rolling pressure nips, and calendering said web bypassing it through said rolling pressure nips.

3. A process as defined in claim 2, in which the cylindrical rollshaving hard metallic surfaces are heated to increase paper temperatureand thereby increase its plasticity and ease of calendering.

4. A process for producing newsprint which comprises, preparing anewsprint pulp furnish of fibers of which at least about one-quarter byweight of the mechanical pulp content is of supergroundwood pulp fibers,making a paper web from said furnish, drying said web, and then,calendering opposite sides of the web through calendering nips formedfrom mutually-reversed resilient and hard calendering rolls.

5. A process as defined in claim 4, in which the newsprint pulp furnishis entirely of supergroundwood pulp fibers.

6. A process, as defined in claim 4, in which said hard rolls are formedof a metal, and said resilient roll is formed by a roll covered by aresilient material.

7. Newsprint produced by the method defined in claim 5.

8. The method of producing a newsprint web with a minimum chemical pulpcontent comprising the steps of z (a) preparing a newsprint furnishcontaining a blend of pulp fibers including supergroundwood to theextent of a least 25% by weight of the total mechanical pulp content ofsaid blend of pulp fibers in which the supergroundwood has a normalshive content;

14 (b) forming a continuous wet web from said furnish by dewatering anaqueous suspension of the blended [furnish and wet pressing the web onpaper making apparatus; (c) drying the wet web in dryer apparatus andattaining normal, controlled moisture content; and (d) calendering thedried web with resilient calender ing means by progressively calenderingopposite sides of the continuous web through calendering nips formedfrom mutually-reversed resilient and hard calendering rolls so that eachside of said web is contacted by a hard roll at least once andcontrolling the soft-calendering lineal pressure and substantiallypreventing the shives from severing local fibers sure rounding theshives for producing an acceptable newsprint with improved burst andtear factors and minimizing shive flaws. 9. Newsprint produced by themethod as claimed in claim 8.

10. The method according to the step set forth in claim 8 in which saidnewsprint furnish is prepared so that the blend contains supergroundwoodto the extent of by weight of the total mechanical pulp content.

11. Newsprint produced by the method as set forth in claim 10.

References Cited UNITED STATES PATENTS 3,124,504 3/1964 Mahoney l62--2063,230,867 l/1966 Nelson 100-93 3,365,774 1/1968 Kusters 29132 OTHERREFERENCES Neill, Superground Wood, Pulp and Paper Magazine of Canada,vol. 64, July 1963, T--229.

MacMillan Shives in Newsprint, Pulp and Paper Magazine of Canada, vol,66, July 1965, T-361.

Gavelin, Fourdrinier Papermaking," (1963), p. 137.

S. LEON BASHORE, Primary Examiner P. CHIN, Assistant Examiner U.S. Cl.X.R. 162-450, 206

